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<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  {yx:if ('a'=='b')}
    {yx:2if('a'=='c')}
      内容1
    {yx:2else}
      内容2
    {/yx:2if}
  {yx:else}
    内容3
  {/yx:if}
</body>
</html>